The present invention relates to an electrophotographic apparatus and a method of forming an electrophotographic image, particularly, to an electrophotographic apparatus and a method of forming an electrophotographic image for forming an image by using a liquid developer.
A wet electrophotographic apparatus using a liquid developer containing a toner and a solvent produces various merits that cannot be produced by a dry electrophotographic apparatus using a developing powder. For example, a very fine toner of sub-micron order can be used in a wet electrophotographic apparatus, making it possible to realize a high image quality. Also, since a sufficiently high image density can be obtained with a small amount of the toner, the wet electrophotographic apparatus is advantageous in economy. Also, the apparatus permits achieving a texture equivalent to that of an offset printing. Further, the toner can be fixed to a paper sheet at a relatively low temperature, leading to an energy saving.
However, some essential problems remain unsolved in the wet electrophotographic apparatus. A first problem inherent in the wet electrophotographic apparatus is derived from the use of a petroleum-based solvent, which exhibits a high resistivity or insulating properties, as a carrier solvent of the liquid developer. The problems derived from the use of the particular liquid developer will now be described with reference to FIGS. 1 and 2.
Specifically, FIG. 1 schematically shows a conventional wet color electrophotographic apparatus. FIG. 2 shows in a magnified fashion a part of the color electrophotographic apparatus shown in FIG. 1.
In the wet color electrophotographic apparatus shown in FIG. 1, a photoconductor drum 1 is housed in an enclosure 100. Four sets of chargers 2-n, laser light exposure sections 3-n, developing devices including developing rollers 10-n and solvent recovery devices including solvent recovery rollers 20-n, which correspond to four colors of yellow, magenta, cyan and black, are sequentially arranged about the outer surface of the photoconductor drum 1. In each set of these four colors, the charger, the laser, the developing device, and the solvent recovery device are arranged in the order mentioned in the direction denoted by an arrow 17. For simplification, the transfer section, etc. are omitted from the drawing of FIG. 1. Also, the developing rollers 10-n alone are depicted in the drawing concerning the developing devices. Further, the solvent recovery rollers 20-n and cleaning blades 21-n alone are depicted in the drawing concerning the solvent recovery devices.
As shown in FIG. 2, the developing rollers 10-n are arranged only slightly apart from the photoconductor drum 1. Also, the developing rollers 10-n are partially dipped in a liquid developer housed in a developing agent reservoir (not shown).
The liquid developer is supplied to an image-holding surface 14 of the photoconductor drum 1 by rotating the developing rollers 10-n in a direction opposite to the rotating direction of the photoconductor drum 1, as denoted by arrows in FIG. 2. Incidentally, the drum 1 is rotated in a clockwise direction; whereas, the developing rollers 10-n are rotated in a counterclockwise direction. In short, drum 1 and the rollers 10-n are opposite to each other in the rotating direction, as described above. Since the developing rollers 10-n are rotated in a direction opposite to the rotating direction of the photoconductor drum 1, the liquid developer 11 within the developing agent reservoir is taken up by the developing rollers 10-n so as to be supplied onto the image-holding surface 14 of the photoconductor drum 1.
In this step, a bias voltage of the polarity equal to that of the charged polarity of the toner 12 is applied to the developing rollers 10-n. Also, an electrostatic latent image is formed in advance on the surface of the photoconductor drum 1 by the chargers 2-n and the laser light exposure sections 3-n. Therefore, an electric field is formed in the liquid developer positioned between the photoconductor drum 1 and the developing rollers 10-n to permit a toner 12 to be moved toward the photoconductor drum 1 by electrophoresis. As a result, a developing agent image is formed on the image-holding surface 14 of the photoconductor drum 1 in a pattern corresponding to the electrostatic latent image.
It should be noted that a film of the solvent containing floating toners 12 that do not contribute to the formation of the developing agent image is formed on the image-holding surface 14 having the image of the developing agent formed thereon. The solvent recovery devices including the recovery rollers 20-n and the blades 21-n are mounted for removing the film of the solvent from the image-holding surface 14.
Like the developing rollers 10-n, the recovery rollers 20-n are arranged only slightly apart from the photoconductor drum 1, as shown in FIG. 2. The solvent film is removed from the image-holding surface 14 of the photoconductor drum 1 by rotating the rollers 20-n in a direction equal to the rotating direction of the drum 1 at a speed as high as possible while applying a bias voltage opposite in polarity to the charged polarity of the toner 12 to the recovery rollers 20-n. In other words, the surface of the recovery roller 20-n is moved as fast as possible in the reverse direction to the moving direction of the surface of the photoconductor drum 1 in a closest point to the photoconductor drum 1. To be more specific, the solvent on the image-holding surface 14 and the toner 12 floating in the solvent are taken up by the recovery rollers 20-n by a mechanism opposite to that described previously in conjunction with the developing rollers 10-n so as to be removed from the image-holding surface 14. The solvent and the toner 12 taken up by the recovery rollers 20-n are scraped off by the blades 21-n so as to be recovered in a recovery vessel (not shown).
It was customary in the past to remove the excess solvent on the image-holding surface 14 of the photoconductor drum 1 by the method described above. In the conventional method, however, it was difficult to remove sufficiently the excess solvent on the image-holding surface 14. As a result, a difficulty was brought about that a film of the excess solvent accompanying formation of a developing agent image of a certain color impairs the formation of an electrostatic latent image by irradiation of the image-holding surface 14 with a laser light. In this case, formation of a developing agent image of another color is impaired. Also, in the conventional method, a film of the excess solvent accompanying formation of a developing agent image of a certain color remains on the image-holding surface 14 in forming an image of the developing agent of another color, giving rise to a color mixing. In short, it is difficult to achieve a satisfactory image quality by the conventional method.
In addition, where a large amount of the excess solvent remains on the image-holding surface 14, the solvent excessively permeates into a paper sheet when the developing agent image is transferred onto the paper sheet. As a result, the image quality is lowered and, at the same time, a bad odor is generated from the paper sheet. It should be noted that, in view of environmental issues, release of the solvent from within the electrophotographic apparatus to the outside should be suppressed as much as possible. Under the circumstances, an additional problem is brought about that a mechanism for removing the solvent permeating the paper sheet or for decreasing the amount of the solvent permeating the paper sheet incurs an increased burden.
An object of the present invention is to provide an electrophotographic apparatus and a method of forming an electrophotographic image capable achieving a satisfactory image quality.
Another object of the present invention is to provide an electrophotographic apparatus that permits suppressing the attachment of a solvent to a paper sheet.
Another object of the present invention is to provide a color electrophotographic apparatus that permits achieving a satisfactory image quality.
Further, still another object of the present invention is to provide a color electrophotographic apparatus that permits suppressing the attachment of a solvent to a paper sheet.
According to a first aspect of the present invention, there is provided an electrophotographic apparatus, comprising: an image holder having a movable image-holding surface; a latent image forming device forming a latent image on the image-holding surface, a developing device forming an image of a developing agent on the image-holding surface by supplying a liquid developer containing a toner and a solvent onto the image-holding surface having the latent image formed thereon, and a solvent recovery device including a solvent recovery surface recovering at least partially the solvent from the image-holding surface, the solvent recovery surface being apart from the image-holding surface and in contact with the solvent attached to the image-holding surface in a position closest to the image-holding surface and moving in a direction opposite to a moving direction of the image-holding surface at the position closest to the image-holding surface, the latent image forming device, the developing device and the solvent recovery device facing the image-holding surface in the order mentioned in the moving direction of the image-holding surface; a transfer unit facing the image-holding surface and transferring the developing agent image from the image-holding surface onto a transfer material; and a control unit connected to the solvent recovery device and controlling the moving speed of the solvent recovery surface at 1 to 4 times as high as the moving speed of the image-holding surface.
According to a second aspect of the present invention, there is provided a color electrophotographic apparatus, comprising: an image holder having a movable image-holding surface; a plurality of developing agent image forming units each comprising latent image forming device forming a latent image on the image-holding surface, a developing device forming an image of a developing agent on the image-holding surface by supplying a liquid developer containing a toner and a solvent onto the image-holding surface having the latent image formed thereon, and a solvent recovery device including a solvent recovery surface and recovering at least partially the solvent from the image-holding surface, the solvent recovery surface being apart from the image-holding surface and in contact with the solvent attached to the image-holding surface in a position closest to the image-holding surface, moving in a direction opposite to a moving direction of the image-holding surface at the position closest to the image-holding surface, and having a curvature radius of 5 mm to 11 mm, the latent image forming device, the developing device and the solvent recovery device facing the image-holding surface in the order mentioned in the moving direction of the image-holding surface; a transfer unit facing the image-holding surface and transferring the developing agent image from the image-holding surfaces onto a transfer material; and a control unit connected to the solvent recovery device and controlling the moving speed of the solvent recovery surface at 1 to 4 times as high as the moving speed of the image-holding surface.
According to a third aspect of the present invention, there is provided a method of forming an electrophotographic image, comprising the steps of:
forming an electrostatic latent image on an image-holding surface being moving; supplying a liquid developer containing a toner and a solvent onto the image-holding surface having the latent image formed thereon to form a developing agent image; moving a solvent recovery surface in a direction opposite to the moving direction of the image-holding surface at a speed 1 to 4 times as high as the moving speed of the image-holding surface such that the solvent recovery surface is apart from the image-holding surface and in contact with the solvent attached to the image-holding surface in a position closest to the image-holding surface, thereby recovering at least partially the solvent from the image-holding surface; and transferring the developing agent image from the image-holding surface onto a transferring material.
According to a fourth aspect of the present invention, there is provided a method of forming an electrophotographic image, comprising the steps of forming a first electrostatic latent image on an image-holding surface being moving; supplying a first liquid developer containing a toner and a solvent onto the image-holding surface having the first latent image formed thereon to form a first developing agent image; forming a second electrostatic latent image on the image-holding surface; supplying a second liquid developer differing in color from the first liquid developer and containing a toner and a solvent onto the image-holding surface having the second latent image formed thereon to form a second developing agent image; moving a solvent recovery surface in a direction opposite to the moving direction of the image-holding surface at a speed 1 to 4 times as high as the moving speed of the image-holding surface such that the solvent recovering surface is apart from the image-holding surface and in contact with the solvent attached to the image-holding surface in a position closest to the image-holding surface, thereby recovering at least partially the solvent from the image-holding surface; and transferring at least one of the first and second developing agent images from the image-holding surface onto a transferring material.
In the present invention, a ratio of the moving speed of the solvent recovery surface to the moving speed of the image-holding surface is set to fall within a predetermined range so as to remove sufficiently the excess solvent attached to the image-holding surface. Therefore, if the present invention is applied to the formation of a color electrophotographic image, an electrostatic latent image can be formed clearly on the image-holding surface, making it possible to prevent the color mixing problem satisfactorily.
In the present invention, the excess solvent is removed from the image-holding surface before the transfer step, making it possible to prevent the solvent from being supplied excessively from the image-holder to the transferring unit. This makes it possible to prevent an excessive amount of the solvent from permeating the paper sheet. It follows that the present invention makes it possible to prevent the image quality from being lowered, to prevent an bad odor from being generated from the paper sheet, and to suppress the burden given to a mechanism for removing the solvent permeating the transfer paper sheet or for decreasing the amount of the solvent permeating the transfer paper sheet.
To reiterate, the excess solvent attached to the image-holding surface is removed sufficiently only by setting a ratio of the moving speed of the solvent recovery surface to the moving speed of the image-holding surface to fall within a predetermined range. Therefore, a residual solvent can be removed sufficiently from the image-holding surface without markedly modifying the construction of the conventional electrophotographic apparatus.
In the present invention, it is desirable to set the curvature radius of the solvent recovery surface at the position closest to the image-holding surface at 5 mm to 11 mm. Where the curvature radius is set to meet this condition, the excess liquid developer can be removed more effectively from the image-holding surface. It is also desirable for the distance between the image-holding surface and the solvent recovery surface at the closest position to fall within a range of between 25 xcexcm and 100 xcexcm. In this case, the excess liquid developer can be removed more effectively from the image-holding surface without affecting the developing agent image formed on the image-holding surface.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.